Applications
|
Chapter 8: Potential Energy and Conservative Forces Applications |
|
FRICTION - Friend or Foe?
Friction is still the "traditional" way to hold things together. A nail is held in place by friction, nuts and bolts would separate if it weren't for friction. Car tires hug the road via friction, various parts in a transmission move together held by friction, wheel assemblies are held together by frictional forces between belts and moving wheels, the list can go on and on. Have you ever considered the fact that the threads in the fabric you are wearing would separate if it weren't for friction. They are not glued together, they are held solely by friction.
The work done against friction is usually of no use whatever and must be dealt with as waste heat. Sometimes, of course, friction-generated heat is exactly the desired effect, e.g. starting a fire by rubbing wood. In most applications, however, much effort goes into reducing friction generated work as much as possible, both to save energy and to reduce wear on the moving parts.
The invention of the wheel was one of the
first victories in the battle against friction losses. Rolling reduces friction related energy loss to
a minimum, ideally to zero, but it also relies heavily on static friction to keep the point on the
ground from moving. Old civilizations, the Egyptians, the Greeks, the Chinese, all developed fairly
sophisticated tribological devices. Wheeled wagons, potters wheels, and other rotating devices
required the development of bearings and lubrication. We know from the pictures they left us that they
developed rudimentary bearings. A bearing is a device that reduces frictional losses as surfaces
side past one another. Engineers distinguish several important types of bearings. A slide
bearing, as the name implies facilitates linear motion between a load and a support (picture on
the left.) A journal bearing permits a load (such as a wheel) to exert radial pressure on a
shaft without excessive energy loss (picture on the right.) 
A thrust bearing pictured below facilitates rotation while the load exerts an axial force
on the shaft.
The yellow markings on these denote spots where lubrication is required. There are several ways to address the lubrication issue. Dry bearings permit the moving surfaces to rub together without any lubricant. The rubbing materials must have intrinsically low friction and low wear properties. Usually the two materials are different and one of them is non-metallic. Treatments or coatings often improve their tribological properties. A dry bearing may contain a metal sleeve that can be replaced as it wears out. In liquid film bearingsthe moving surfaces are separated by a lubricating liquid or gas film. In modern bearings the pressure, necessary for the separation of the moving surfaces, is produced hydrodynamically as a result of the motion of the fluid. A lot of fancy physics goes into the engineering of such bearings. Yet another category are rolling bearings.In a rolling bearing the wheel and the shaft do not come in direct sliding contact. Rather, the wheel rides on moving parts, balls or cylinders, which roll on the shaft.
Ancient Greeks and Romans developed wheels, levers, gears
and pulleys to aid their construction efforts in land structures and shipbuilding. These devices
depend on good bearing and pivot designs. During the middle ages people derived their power from water
and wind sources. Large water and wind mills were built, turning on wooden shafts which rubbed against
wooden or stone bearing blocks, lubricated with tallow or lard. The flourishing of mechanical
construction in the 15th and 16th centuries required that some thought be given to problems of
friction and wear. A sizable portion of Leonardo da Vinci's drawings deal with the scientific
investigation of bearing materials and friction. Da Vinci understood the benefits of a rolling
bearing.
During the 17th century, the times of Isaac Newton, the "laws of friction" which we still use were stated. Isaac Newton proposed that lubrication depends on the fact that fluids resist flow ( the phenomenon later labeled viscosity.) in particular, that this resistance depends on the velocity gradient. A century and a half later a French mathematician Claude Navier developed the mathematical theory of fluid flow and introduced the coefficient of viscosity. In the late 1700, Charles Coulomb undertook a comprehensive study of friction. He distinguished between the effect of adhesion, binding of one surface to another, and deformation, the interlocking of surface irregularities. Coulomb thought that the latter played a more important role in friction than the former.
While the details of friction are complicated and far from understood, the basic principles have been kown for some time. These principles can be summarized in two empirical laws of friction which were first proposed in 1699 by Guillaume Amontons, a French engineer. The first principle states that the force of friction between two sliding surfaces is proportional to the normal force between them. The second principle states that the force does not depend on the area of contact. The notion of area of contact is not very precise. The surfaces are never perfectly flat. They contain imperfections and impurities. Thus the true area of contact depends o these properties. So does the coefficient of friction. The coefficient of friction of a good tire on a dry road is about 1. On a wet road it may be less than half of that.
The mathematics of interlocking irregularities was first investigated by the Swiss mathematician Leonhard Euler, who introduced the Greek symbol mu for the coefficient of friction. During sliding the two surfaces deform each other elastically (no loss of energy) of plastically. Modern tribology explains Amontons second principle as the consequence of the statistical distribution of these surface characteristics.
2.
3.
5.
6.
7.
A Pearson Company Distance Learning at Prentice Hall Legal Notice |